Colonial Marine Invertebrates Research Articles

Intracolonial genetic variation in the scleractinian coral Seriatopora hystrix

In recent years, increasing numbers of studies revealed intraorganismal genetic variation, primarily in modular organisms like plants or colonial marine invertebrates. Two underlying mechanisms are distinguished: Mosaicism is caused by somatic mutation, whereas chimerism originates from allogeneic fusion. We investigated the occurrence of intracolonial genetic variation at microsatellite loci in five natural populations of the scleractinian coral Seriatopora hystrix on the Great Barrier Reef. This coral is a widely distributed, brooding species that is at present a target of intensive population genetic research on reproduction and dispersal patterns. From each of 155 S.hystrix colonies, either two or three samples were genotyped at five or six loci. Twenty-seven (~17%) genetically heterogeneous colonies were found. Statistical analyses indicated the occurrence of both mosaicism and chimerism. In most cases, intracolonial variation was found only at a single allele. Our analyses suggest that somatic mutations present a major source of genetic heterogeneity within a single colony. Moreover, we observed large, apparently stable chimeric colonies that harbored clearly distinct genotypes and contrast these findings with the patterns typically observed in laboratory-based experiments. We discuss the error that mosaicism and chimerism introduce into population genetic analyses.

Histocompatibility in an invertebrate is controlled by a complex of polymorphic IgSF-like genes. (170.4)

Abstract Animals have evolved sophisticated immune systems to distinguish self from infectious non-self. In addition, several groups of colonial marine invertebrates—animals such as sponges, corals, and sea squirts—have evolved allorecognition systems that allow them to distinguish between their own tissues and those other members of their species. Allorecognition phenomena occur when these animals, which live as encrustations on marine surfaces, grow into contact with each other. Compatible colonies fuse, while incompatible colonies reject and often compete for space. Although allorecognition phenomena have long been known to be under genetic control, the molecular basis of these phenomena remains largely unknown. One model system for the study of invertebrate allorecognition is Hydractinia, a cnidarian related to Hydra and reef corals. Using lines of Hydractinia inbred for >8 generations, we have mapped allorecognition loci to a single genomic interval, called the allorecognition complex (ARC). The ARC contains at least two histocompatibility genes, alr1 and alr2, each of was identified by positional cloning. Both genes encode polymorphic transmembrane proteins similar to immunoglobulin superfamily (IgSF) molecules. Sequence variation between alleles of alr1 or alr2 is concentrated in the extracellular region and is predictive of allorecognition responses in inbred and wild-type colonies. Intriguingly, alr1 is located within a family of at least 10 other IgSF-like genes.

Model Systems of Invertebrate Allorecognition

Nearly all colonial marine invertebrates are capable of allorecognition--the ability to distinguish between self and genetically distinct members of the same species. When two or more colonies grow into contact, they either reject each other and compete for the contested space or fuse and form a single, chimeric colony. The specificity of this response is conferred by genetic systems that restrict fusion to self and close kin. Two selective pressures, intraspecific spatial competition between whole colonies and competition between stem cells for access to the germline in fused chimeras, are thought to drive the evolution of extensive polymorphism at invertebrate allorecognition loci. After decades of study, genes controlling allorecognition have been identified in two model systems, the protochordate Botryllus schlosseri and the cnidarian Hydractinia symbiolongicarpus. In both species, allorecognition specificity is determined by highly polymorphic cell-surface molecules, encoded by the fuhc and fester genes in Botryllus, and by the alr1 and alr2 genes in Hydractinia. Here we review allorecognition phenomena in both systems, summarizing recent molecular advances, comparing and contrasting the life history traits that shape the evolution of these distinct allorecognition systems, and highlighting questions that remain open in the field.

Life in the Colonies: Learning the Alien Ways of Colonial Organisms

Who needs to go to outer space to study alien beings when the oceans of our own planet abound with bizarre and unknown creatures? Many of them belong to sessile clonal and colonial groups, including sponges, hydroids, corals, octocorals, ascidians, bryozoans, and some polychaetes. Their life histories, in many ways unlike our own, are a challenge for biologists. Studying their ecology, behavior, and taxonomy means trying to “think like a colony” to understand the factors important in their lives. Until the 1980s, most marine ecologists ignored these difficult modular organisms. Plant ecologists showed them ways to deal with the two levels of asexually produced modules and genetic individuals, leading to a surge in research on the ecology of clonal and colonial marine invertebrates. Bryozoans make excellent model colonial animals. Their life histories range from ephemeral to perennial. Aspects of their lives such as growth, reproduction, partial mortality due to predation or fouling, and the behavior of both autozooids and polymorphs can be studied at the level of the colony, as well as that of the individual module, in living colonies and over time.

Character lability in deep-sea bamboo corals (Octocorallia, Isididae, Keratoisidinae)

Morphological traits that appear multiple times when mapped onto molecular phyloge- nies have been associated with character lability. In an ecological context, functional characters, if labile, could confer advantages for adaptation to specific habitats. Bamboo corals are long-lived, deep-sea octocorals characterized by an obvious modularity, which affords diverse branching mor- phologies in the Keratoisidinae subfamily. We reconstructed molecular phylogenies using 16S (mito- chondrial) and ITS2 (nuclear) ribosomal DNA, and obtained 39 ITS2 and 22 16S sequences from 22 different specimens. The molecular topologies showed Keratoisidinae genera as polyphyletic. Twelve morphological characters were chosen to make the ancestral character reconstruction, none of which exhibited character states forming monophyletic groups when mapped onto molecular phy- logenies. The different character states appeared as having been gained and lost multiple times. Modular organisms such as bamboo corals could have higher character evolvability than unitary organisms. Complexity arising from simple body plans is a typical characteristic in most modular organisms, such as plants and colonial marine invertebrates. However, bamboo corals may also exhibit phenotypic plasticity associated with continuous characters, given that they can respond to extrinsic controls. Our results also have direct repercussions on Isididae taxonomy, since morpho- logical characters have been used for their classification. Consequently, we suggest that the current taxonomy of the group is re-evaluated and that the different mechanisms regulating modularity and character evolution in bamboo corals are explored.

Selection on offspring size among environments: the roles of environmental quality and variability

1. How mothers balance the trade-off between offspring size and number to maximize maternal fitness has long been of interest to ecologists seeking to understand the evolution of offspring size. Predictions of the optimal offspring size depend fundamentally on the relationship between offspring size and offspring performance, which may in turn vary with environmental conditions. 2. Selection for larger offspring is expected to intensify as environmental quality deteriorates. Models also predict that variable selection on offspring size may favour the evolution of larger offspring than those favoured when selection is constant, or of strategies of variable offspring provisioning (e.g. bet-hedging, plasticity). To date, there is mixed empirical support for the first expectation and few tests of the second. Given, however, that offspring size effects are often estimated under controlled laboratory conditions that presumably downplay their strength and variability, we may not yet understand how selection shapes offspring size in nature. 3. We examined several relationships between offspring size and performance in controlled (laboratory) and natural (field) environments over time for a colonial marine invertebrate, Bugula neritina, and assessed the variability of these relationships by doing so for replicate cohorts. We further developed a simple optimality model to examine whether predictions of the optimal offspring size were similar (or similarly variable) across environments. 4. We found that selection on offspring size varied substantially among laboratory and field environments, and among cohorts in the latter. In the laboratory, our model consistently predicted that mothers should maximize their fecundity by producing the smallest possible offspring. In the field, however, the predicted optimal offspring size varied from the smallest possible size to the largest possible size for different cohorts. 5. Our study suggests that laboratory estimates of offspring size effects, though often necessary, may not always reflect the direction or variability of selection on offspring size under natural conditions. The optimal offspring size for mothers in nature may be an ever-shifting target that shapes provisioning strategies such as bet-hedging or plasticity in offspring size.

A Hypervariable Invertebrate Allodeterminant

Colonial marine invertebrates, such as sponges, corals, bryozoans, and ascidians, often live in densely populated communities where they encounter other members of their species as they grow over their substratum. Such encounters typically lead to a natural histocompatibility response in which colonies either fuse to become a single, chimeric colony or reject and aggressively compete for space. These allorecognition phenomena mediate intraspecific competition, support allotypic diversity, control the level at which selection acts, and resemble allogeneic interactions in pregnancy and transplantation. Despite the ubiquity of allorecognition in colonial phyla, however, its molecular basis has not been identified beyond what is currently known about histocompatibility in vertebrates and protochordates. We positionally cloned an allorecognition gene by using inbred strains of the cnidarian, Hydractinia symbiolongicarpus, which is a model system for the study of invertebrate allorecognition. The gene identified encodes a putative transmembrane receptor expressed in all tissues capable of allorecognition that is highly polymorphic and predicts allorecognition responses in laboratory and field-derived strains. This study reveals that a previously undescribed hypervariable molecule bearing three extracellular domains with greatest sequence similarity to the immunoglobulin superfamily is an allodeterminant in a lower metazoan.

Multiple paternity and subsequent fusion-rejection interactions in a kin-structured population

Although numerous studies have explored the ecological and evolutionary conse- quences of relatedness on interactions among individuals, few have explored how variation in multi- ple paternity in natural populations affects subsequent interactions at later life history stages. Addi- tional fathers result in a more genetically diverse brood because the ratio of half siblings to full siblings is increased. For sessile, colonial marine invertebrates, which typically have limited larval dispersal, the probability of a juvenile fusing with a neighboring conspecific is likely to be affected by the genetic composition of the brood from which it is derived. Consequently, we explored the rela- tionship between multiple paternity and subsequent fusion-rejection interactions in the colonial ascidian Botryllus schlosseri. Microsatellites were used to calculate an effective paternity index for each brood and fusion tests among juveniles collected from recruitment plates adjacent to maternal colonies were used to assay fusion frequency. Results support the hypothesis that fusion frequency among recruits decreases with increasing levels of multiple paternity, indicating that fertilization pro- cesses affect interactions among later life stages. By extension, fertilization outcomes that affect lev- els of multiple paternity should be incorporated into studies of juvenile and adult interactions in other taxa with kin-structured populations.

The origins of graptolites and other pterobranchs: a journey from ‘Polyzoa’

The graptolites, known only from fossils, have been convincingly allied to the pterobranch hemichordates, a group of tiny, mostly colonial marine invertebrates bearing feeding arms. The phylogenetic position of pterobranchs has been subject to debate and revision for over a century. Their colonial lifestyle and feeding arms were originally seen as evidence placing them among the bryozoans, until later and more careful anatomical studies revealed more characters in common with acorn worms. Pterobranchs and acorn worms are now grouped as the phylum Hemichordata. For many decades, it was thought that pterobranchs were closer to the ancestral form of hemichordates, particularly because ‘lophophorate’ invertebrates also possess feeding arms, notably the phoronids and bryozoans, as do crinoid echinoderms. This traditional view has been challenged by recent molecular evidence. First, there is strong molecular evidence to indicate that lophophorates are very distant from hemichordates and echinoderms, in a different major branch of the animal phylogenetic tree. Therefore, similarities between the feeding structures must be due to convergent evolution. Second, there is strong evidence that hemichordates and echinoderms form a clade (Ambulacraria) within the deuterostomes, rather than hemichordates being closer to chordates. Third, there is weaker evidence that pterobranchs may be derived from acorn worms, and hence that the vermiform body plan may be ancestral within hemichordates. This suggestion warrants further testing. Here we review the evidence for these conclusions, highlight strengths and weaknesses in the data and analyses, and consider the implications for the origins of pterobranchs and graptolites.

Gorgonian mortality related to a massive attack by caprellids in the Bunaken Marine Park (North Sulawesi, Indonesia)

A massive attack of caprellids is reported here, that is related to a local mortality event of gorgonians in North Sulawesi. Three species of sea fans were affected by the presence of Metaprotella sandalensis, a caprellid widely distributed in the Indo-Pacific. The degree of damage here documented was in relation to the skeletal features of the gorgonian species. The amphipod gut contents were analysed, highlighting an unusual trophic source for caprellids and a new predator for gorgonians. This phenomenon is discussed also evidencing parallels between colonial marine invertebrates and their predators and terrestrial plant–herbivore interactions.

The Fetal Allograft Revisited: Does the Study of an Ancient Invertebrate Species Shed Light on the Role of Natural Killer Cells at the Maternal-Fetal Interface?

Human pregnancy poses a fundamental immunological problem because the placenta and fetus are genetically different from the host mother. Classical transplantation theory has not provided a plausible solution to this problem. Study of naturally occurring allogeneic chimeras in the colonial marine invertebrate, Botryllus schlosseri, has yielded fresh insight into the primitive development of allorecognition, especially regarding the role of natural killer (NK) cells. Uterine NK cells have a unique phenotype that appears to parallel aspects of the NK-like cells in the allorecognition system of B. schlosseri. Most notably, both cell types recognize and reject “missing self” and both are involved in the generation of a common vascular system between two individuals. Chimeric combination in B. schlosseri results in vascular fusion between two individual colonies; uterine NK cells appear essential to the establishment of adequate maternal-fetal circulation. Since human uterine NK cells appear to de-emphasize primary immunological function, it is proposed that they may share the same evolutionary roots as the B. schlosseri allorecognition system rather than a primary origin in immunity.

Social Evolution: The Decline and Fall of Genetic Kin Recognition

Animals should benefit from the ability to recognise their kin, yet curiously this faculty is often absent. New theory confirms that genetic kin recognition is inherently unstable, explaining its rarity.

Embryonic chimerism does not induce tolerance in an invertebrate model organism.

In colonial marine invertebrates, allorecognition restricts somatic fusion and thus, chimerism, to histocompatible individuals. Little is understood, however, about how invertebrates respond to chimerism formed across histocompatibility barriers or whether embryonic exposure to histoincompatible cells induces allotolerance. We here evaded natural allorecognition barriers by generating well mixed embryonic chimeras of Hydractinia symbiolongicarpus (Cnidaria:Hydrozoa) and developed molecular markers to detect chimerism in both histocompatible and histoincompatible settings. Histocompatible chimeras exhibited markedly higher growth rates and survivorship than histoincompatible pairings. Histoincompatible chimeras were unstable, with chimerism being undetectable by 4 wk of age. In contrast, colonies generated from histocompatible pairings remained chimeric at markedly higher frequencies and longer durations. Histoincompatible chimeras that lost detectable chimerism retained the fusibility/rejection characteristics of the remaining component of the chimera but not that of the lost component. Chimerism across histocompatibility barriers in an invertebrate model organism was unstable and did not induce tolerance.

OFFSPRING SIZE EFFECTS MEDIATE COMPETITIVE INTERACTIONS IN A COLONIAL MARINE INVERTEBRATE

Over the past 30 years, numerous attempts to understand the relationship between offspring size and fitness have been made, and it has become clear that this critical relationship is strongly affected by environmental heterogeneity. For marine invertebrates, there has been a long-standing interest in the evolution of offspring size, but there have been very few empirical and theoretical examinations of post-metamorphic offspring size effects, and almost none have considered the effect of environmental heterogeneity on the offspring size/fitness relationship. We investigated the post-metamorphic effects of offspring size in the field for the colonial marine invertebrate Botrylloides violaceus. We also examined how the relationship between offspring size and performance was affected by three different types of intraspecific competition. We found strong and persistent effects of offspring size on survival and growth, but these effects depended on the level and type of intraspecific competition. Generally, competition strengthened the advantages of increasing maternal investment. Interestingly, we found that offspring size determined the outcome of competitive interaction: juveniles that had more maternal investment were more likely to encroach on another juvenile's territory. This suggests that mothers have the previously unrecognized potential to influence the outcome of competitive interactions in benthic marine invertebrates. We created a simple optimality model, which utilized the data generated from our field experiments, and found that increasing intraspecific competition resulted in an increase in predicted optimal size. Our results suggest that the relationship between offspring size and fitness is highly variable in the marine environment and strongly dependent on the density of conspecifics.

OFFSPRING SIZE AFFECTS THE POST-METAMORPHIC PERFORMANCE OF A COLONIAL MARINE INVERTEBRATE

The positive relationship between offspring size and offspring fitness is a fundamental assumption of life-history theory, but it has received relatively little attention in the marine environment. This is surprising given that substantial intraspecific variation in offspring size is common in marine organisms and there are clear links between larval experience and adult performance. The metamorphosis of most marine invertebrates does not represent a newbeginning, and larval experiences can have effects that carry over to juvenile survival and growth. We show that larval size can have equally important carryover effects in a colonial marine invertebrate. In the bryozoan Bugula neritina, the size of the non-feeding larvae has a prolonged effect on colony performance after metamorphosis. Colonies that came from larger larvae survived better, grew faster, and reproduced sooner or produced more embryos than colonies that came from smaller larvae. These effects crossed generations, with colonies from larger larvae themselves producing larger larvae. These effects were found in two populations (in Australia and in the United States) in contrasting habitats.

Variation in the dispersal potential of non-feeding invertebrate larvae: the desperate larva hypothesis and larval size

For many species of marine invertebrates, variability in larval settlement behaviour appears to be the rule rather than the exception. This variability has the potential to affect larval dispersal, because settlement behaviour will influence the length of time larvae are in the plankton. Despite the ubiquity and importance of this variability, relatively few sources of variation in larval settlement behaviour have been identified. One important factor that can affect larval settlement behaviour is the nutritional state of larvae. Non-feeding larvae often become less discriminating in their 'choice' of settlement substrate, i.e. more desperate to settle, when energetic reserves run low. We tested whether variation in larval size (and presumably in nutritional reserves) also affects the settlement behaviour of 3 species of colonial marine invertebrate larvae, the bryozoans Bugula ner- itina and Watersipora subtorquata and the ascidian Diplosoma listerianum. For all 3 species, larger larvae delayed settlement for longer in the absence of settlement cues, and settlement of Bugula ner- itina larvae was accelerated by the presence of settlement cues, independently of larval size. In the field, larger W. subtorquata larvae also took longer to settle than smaller larvae and were more dis- criminating towards settlement surfaces. These differences in settlement time are likely to result in differences in the distance that larvae disperse in the field. We suggest that species that produce non- feeding larvae can affect the dispersal potential of their offspring by manipulating larval size and thus larval desperation.

Fungal infections of a colonial marine invertebrate: Diversity and morphological consequences

Colonies of the bryozoan speciesHippodiplosia insculpta collected from Grandmother’s Cove (American Camp, San Juan Island, Washington, USA) were analyzed in view of pathologic growth patterns. The species produced giant buds that were filled with extracellular polymeric substances and a dense microbial biofilm consisting of bacteria and fungal hyphae. Fungi were isolated from the colonies and were identified asPenicillium expansum, Peniillium brevicompactum, Sclerotinia sclerotiorum, Acremonium breve, andCladosporium sphaerospermum. The results of this study indicate that the formation of giant buds in the bryozoan is a defense mechanism against fungal infection.

Mate selection and the evolution of highly polymorphic self/nonself recognition genes.

Multicellular organisms use the products of highly polymorphic genes to distinguish self from conspecific nonself cells or tissues. These allorecognition polymorphisms may regulate somatic interactions between hosts and pathogens or between competitors (to avoid various forms of parasitism), as well as reproductive interactions between mates or between gametes (to avoid inbreeding). In both cases, rare alleles may be advantageous, but it remains unclear which mechanism maintains the genetic polymorphism for specificity in self/nonself recognition. Contrary to earlier reports, we show that mate selection cannot be a strong force maintaining allorecognition polymorphism in two colonial marine invertebrates. Instead, the regulation of intraspecific competitive interactions appears to promote the evolution of polymorphisms in these species.

Egg production by colonies of a gorgonian coral

Reproductive success, the production and fertilization of gametes, is a key component of fitness. Among many colonial marine invertebrates, the production of gametes by a colony is a function of both gamete production per module (e.g., polyp, zooid) and the number of modules in the colony (i.e., colony size). We examined variance in gamete production per polyp and egg production per colony over a range of colony sizes, and the relationship between egg production and growth in the common Caribbean gorgonian Plexaura flexuosa. The number of polyps per colony and the average number of mature eggs per polyp both were greater among larger female colonies (>70 cm in height) than among smaller colonies (< 70 cm), resulting in a 1 to 2 order of magnitude increase in whole colony egg release for the larger colonies. In a group of 24 colonies, 98 % of the 9.2 x 10 6 eggs produced in one spawning event came from the 12 colonies taller than 70 cm. Branch extension rates showed no relationship to colony size, but whole colony relative growth appears to decrease as colony size increases. This suggests that proportionately less energy is used for growth as a colony gets larger, and thus may be available for reproduction.

Prudent sessile feeding by the corallivore snail, Coralliophila violacea on coral energy sinks

Convergence of form and function has accompanied the evolution of modular growth in terrestrial plants and colonial marine invertebrates. Part of this convergence is related to the optimal exploitation of resources (space and light) and the ability to translocate energy products from sources to sink sites. Feeding on the energy pathways and energy sinks of terrestrial plants is a well–known phenomenon. Hermatypic corals, the major organisms constructing tropical reef environments, contain photosynthetic algae (zooxanthellae), energetic products of which are translocated towards sink sites located at the coral's growing tips and regenerating areas. Despite the plant–coral convergence in energy pathways and sinks, there has been no evidence to date that coral energy sinks are exploited by coral predators. Gastropods of the genus Coralliophila are found feeding on coral margins, causing small and localized tissue damage. However, the ability of these snails to continue to feed without moving over a long period remains puzzling. Using a 14C labelling technique, we found that colony margins of the stony coral Porites function as major energy sinks. When snails inhabited these sites they incorporated significant amounts of 14C, indicating that they had fed on photosynthetic products translocated from the interior of the colony. Furthermore, when snails aggregate in the interior of the colony, thereby causing large surface injuries, they induce the development of significant new sink sites. This mode of prudent sessile feeding maximizes the efficiency of energy exploitation by the predatory snail, while minimizing tissue damage to the coral. The fact that energy sink sites occur in many coral species suggests that the strategy of sink exploitation for nutrition could also occur in many other marine host–symbiont relationships.